The present invention relates to methods and apparatus for checking the presence of yarns of a tentered yarn layer on a textile machine, by means of a measuring feeler, movable relative to the yarn layer.
Arrangements of this type, when used as warp motion stop devices on weaving machines, can replace the known drop wires of warp motion stop devices. As is known, these drop wires are lined up on the warp yarns and fall onto a contact rail when a yarn breaks, thereby generating an electrical Signal. Since an additional stress on the warp yarns is caused by the drop wires of the warp motion stop device. Moreover, since the warp yarns have to be drawn into the drop wires, there has long been the need for a replacement for the known drop wires of warp motion stop devices. This need could be met in principle by a measuring feeler which moves relative to the yarn layer for sensing individual yarns.
The main requirement of such a measuring feeler arrangement for checking the presence of the yarns of a weaving warp is, of course, the absolutely reliable detection of yarn breaks, but this cannot be guaranteed with purely optical systems. Optical systems (see, for example, DE-A-3,832,984 and U.S. Pat. No. 4,772,800) have therefore been unsuccessful hitherto, and sensing means with mechanical feelers have been proposed. In a sensing means of this mechanical type described, for example, in U.S. Pat. No. 4,525,705, a feeler with a tracer rod is provided, and when the latter is deflected out of a position of equilibrium a circuit is closed and the presence of a yarn is thereby indicated. By counting these yarns consecutively and by comparing the sum with the known number of yarns, the number of yarn breaks, which corresponds to the difference in the two values, can be determined.
Since a sufficiently large deflection of the tracer rod is necessary for a reliable detection of the individual yarns, the arrangement described in U.S. Pat. No. 4,525,705 cannot satisfy the customary requirements for a warp motion stop device, at least where dense warps are concerned.